有关C++继承与友元、继承与类型转换详解

#include <iostream> 

using namespace std; 

class a{
friend class pal;
private:
	int i;
protected:
	int j;
public:
	int k;
};

class b:public a{
protected:
	int l;
};

class c:protected a{};
class d:private a{};
class e:public b{};

class pal{
public:
	void f(a a1){
		cout<<a1.i;//正确，友元能访问私有成员
	}
	void f1(b b1){
		cout<<b1.i;//正确，友元能访问派生类的基类部分
		cout<<b1.l;//出错，pal类不是b的友元
	}
};

class deri_pal:public pal{
public:
	void f2(a a1){
		cout<<a.i;//出错，友元不能继承
	}
};

int main() 
{	
	a a1;
	b b1;
	c c1;
	d d1;
	e e1;
	a1=b1;//正确，只有公共继承的派生类才能向基类转换
	a1=e1;//正确
	a1=c1;//出错，只有公共继承的派生类才能向基类转换
	a1=d1;//出错，只有公共继承的派生类才能向基类转换
	system("pause");
	return 0; 
} 

#include <iostream> 

using namespace std; 

class a{};
class b:public a{};
class c:protected a{};
class d:private a{};
class bb:public b{};
class cc:public c{};
class dd:public d{};




int main() 
{	
	a a1;b b1;c c1;d d1;bb bb1;cc cc1;dd dd1;
	a* a1=&b1;//ok
	a* a2=&c1;//error
	a* a3=&d1;//error
	a* a4=&bb1;//ok
	a* a5=&cc1;//error
	a* a6=&dd1;//error
	system("pause");
	return 0; 
}